This study is aimed at determining whether the failure of human cytomegalovirus (CMV) to replicate in nonpermissive cells is due to the absence of some essential gene product(s) in the nonpermissive cell, or alternatively to the presence of some inhibitory product(s). This question will be approached by examining CMV replication in: 1) hybrid cells derived from one permissive and one nonpermissive parent by polyethylene glycol-induced fusion; 2) heterokaryons derived from permissive and nonpermissive parents prelabeled with different fluorescent probes and purified soon after fusion using the fluorescence-activated cell sorter; and 3) partial hybrids created by fusion between an intact parental cell and a microcell carrying only small numbers of chromosomes. All these cells will be screened for the ability to sustain CMV replication by the following techniques: 1) plaque assay of culture medium; 2) infectious center assays; 3) electron microscopy; 4) DNA hybridization to detect viral DNA synthesis; and 5) indirect immunofluorescence to detect synthesis of viral antigens. We will determine from these studies whether nonpermissiveness behaves as an expressed or repressed trait, and will correlate this with the chromosome composition of the cells as determined by banding techniques and isozyme electrophoresis, in order to identify chromosomes essential for CMV expression or repression.